The largest moon of Saturn is a high priority for planetary exploration. Titan is an ocean world, and the only moon in our Solar System with a dense atmosphere, which supports an Earth-like hydrological cycle of methane clouds, rain, and liquid flowing across the surface to fill lakes and seas. The abundant complex organic material accessible on Titan's surface makes it an ideal destination to study the conditions necessary for the habitability of an extraterrestrial environment and the kinds of chemical interactions that occurred before life developed on Earth.
The two Voyager spacecraft observed Titan in 1979 and 1980, but the organic haze in the atmosphere obscures the surface at visible wavelengths. The Hubble Space Telescope imaged Titan at longer near-infrared wavelengths in 1994, revealing large bright and dark regions on its surface. However, the details of Titan's surface landforms remained a mystery until July 2004, when NASA's Cassini spacecraft arrived. Cassini used radar and imaging at near-infrared wavelengths to glimpse what lies below the hazy atmosphere. During over 100 close flybys, the Cassini orbiter has coarsely mapped much of Titan's surface and made detailed studies of its atmosphere. Cassini also delivered the Huygens probe, which in 2005 parachuted safely down to Titan's surface, measured winds and the atmosphere, and imaged a tiny part of the surface in close-up detail. Cassini's observations also point to a salty ocean beneath its water-ice crust, thus earning Titan the label of an "Ocean World".
Cassini has shown us that Titan's surface has rivers, lakes, and even seas of liquid ethane and methane (the main component of natural gas), as well as vast expanses of sand dunes. The climate of Titan is such that the methane can form clouds and even rain, as water does on Earth. Titan's atmosphere is four times denser than Earth's and it's gravity is about 1/7th of Earth's; this combination means Titan's raindrops fall much slower than Earth's. Rainfall on Titan is rare – it may be centuries between showers at a given location, and Cassini has observed only two definite rainstorms so far.
Titan's atmosphere is composed primarily of nitrogen (~95%), with ~5% of methane and small amounts of other carbon-rich compounds. When exposed to sunlight, the methane and nitrogen molecules are split apart by ultraviolet light and recombine to form a variety of complex organic compounds. Organic molecules are the building blocks for life, and their presence on Titan adds to its intrigue – what compounds are on Titan, and what might they form?
NASA included exploration of Ocean Worlds among potential missions that the agency is seeking through the 2017 New Frontiers Competition. The Ocean Worlds mission theme is focused on the search for signs of extant life and/or characterizing the potential habitability of Titan and/or Enceladus. Titan has a unique combination of abundant, complex, carbon-rich chemistry on the surface of a water-ice-dominated ocean world – ideal for the study of prebiotic chemistry and habitability of an extraterrestrial environment. For Titan, New Frontiers science objectives of the Ocean Worlds mission theme are:
Dragonfly is a rotorcraft lander mission proposed to the New Frontiers Program that is designed to take advantage of Titan's environment to be able to sample materials and determine the surface composition in different geologic settings. This revolutionary mission concept would provide the capability to explore diverse locations to characterize the habitability of Titan's environment, to investigate how far prebiotic chemistry has progressed, and even to search for chemical signatures that could be indicative of water-based and/or hydrocarbon-based life.
The Dragonfly dual-quadcopter could make multiple flights to explore a variety of locations on Titan. The dense, calm atmosphere and low gravity make flying an ideal means to travel to different areas of the moon – studies from the late-1990s onwards identified aerial mobility as a key enabler for Titan exploration, including helicopters, balloons, and airplanes. In a single flight of up to an hour, Dragonfly could fly a few 10s of km, farther than any planetary rover has traveled. With one Dragonfly hop per Titan day (16 Earth days), the rotorcraft could travel from its initial landing site to explore sites several hundred kilometers away within the planned ~2-yr mission duration. Although unique in its ability to fly, Dragonfly would spend most of the time on the surface making science measurements.
Unable to use solar power under Titan's hazy atmosphere, Dragonfly would use a Multi-Mission Radioisotope Thermoelectric Generator (MMRTG) like the Curiosity rover on Mars. Flight, data transmission, and most science operations would be planned during Titan's day (8 Earth days), with plenty of time during the Titan night to recharge.